A voltage regulator converts an unregulated input voltage to a regulated output voltage. A voltage set point determines a magnitude of the regulated output voltage. The voltage regulator includes a control module that compares the voltage set point and the regulated output voltage and adjusts the regulated output voltage in accordance with the comparison.
The control module typically includes an analog-to-digital converter (ADC) having an input that receives the regulated output voltage. The ADC converts the regulated output voltage, which is an analog signal, to a digital value that can be used by the control module. The ADC has an input voltage range and an output resolution that are both generally fixed. For example, a particular ADC may be a 3-comparator design that has an input voltage range of V*+/−δ, where V* is the voltage set point and δ is a maximum output swing of the ADC. This example ADC could then provide a binary count of 00 when the regulated output voltage is below V*−δ, a binary count of 01 when the regulated output voltage is between V*−δ and V*, a binary count of 10 when the regulated output voltage is between V* and V*+δ, and a binary count of 11 when the regulated output voltage is greater than V*+δ.
Referring now to FIG. 1, the regulated output voltage 10 is shown relative to the voltage set point V*. The lower limit of the ADC input voltage range is at V*−δ. The upper limit of the ADC input voltage range is at V*+δ. The voltages are measured with respect to a reference potential, such as ground, that is represented by a reference line 12.
The regulated output voltage 10 should remain between V*−δ and V*+δ for the ADC to generate the binary count that represents the actual magnitude of the regulated output voltage 10. When the regulated output voltage 10 goes above V*+δ or below V*−δ, it is “out of range”. When the regulated output voltage 10 goes out of range, such as is shown at peaks 14-1, 14-2, . . . , 14-5, referred to collectively as the peaks 14, the ADC is unable to provide a binary count that represents the actual magnitude of the regulated output voltage 10. Therefore, during the first peak 14-1, the control module is unable to determine a magnitude of difference between the voltage set point V* and the regulated output voltage 10. The remaining peaks 14-2, . . . , 14-5 show that the control module has become unstable due to the first peak 14-1.
One way to prevent the voltage regulator from becoming unstable when the ADC goes out of range is to use an ADC that has a larger input voltage range. However, increasing the input voltage range generally also requires increasing the ADC output resolution, which causes the ADC to increase in size, cost, and power consumption.